Reliable stacking of paper sheets is a well known problem. Paper sheet stacks of large vertical height tend to curl such that insertion of the next sheet is obstructed. This phenomenon varies as a function of humidity, temperature, and the characteristics of the sheet processing machine. It is particularly troublesome in the art of xerographic machines having a hot roll fuser. This fuser not only heats and drys the sheet, but also tends to "iron" a curl into the sheet.
Various sheet stacking means have been provided in the prior art. For example, air jets and mechanical fingers have been utilized to push the sheets down onto the stack. In high speed card stacking devices, stacking rollers have been provided to engage the stack's trailing edge and to bend the stack's entire trailing edge away from the path of the next card's insertion.
In the art of xerography, collators adapted to stack sheets have been constructed and arranged such that each collator bin includes a flexible entrance flap to hold down the stack's trailing edge so as not to obstruct insertion of the next sheet. Copy exit trays of xerographic machines have been provided with paper feed rolls, one of which includes a long, resilient flap whose function it was to aid in sheet stacking. In addition, original documents fed to the platen of a xerographic copier have been returned to a stacking tray after copying, wherein the tray includes a 180.degree. rotatable two-bar linkage. This linkage includes one bar to hold the stack down for insertion of the next sheet. After insertion the linkage is rotated 180.degree. in preparation for the next sheet.
The present invention advances the art of sheet stacking by a unique relationship of structural elements, including a two-bar hold-down mechanism of unique movement relationship.
The present invention stacks sheets on a vertically adjustable platform. Thus, the troublesome variables, introduced as the relative positions of the sheet's entry path and stacking plane change, are eliminated.
As is well accepted in the art, a hinged wire form, supporting a decelerating roller, is engaged by the sheet's leading edge, as the sheet is stacked. However, since the stack height is controlled at a given position, the angle of attack between the sheet's leading edge and this wire-form is maintained constant, and another troublesome variable is eliminated.
The present invention's two above-mentioned bars comprise a pusher bar and hold-down bar. In the quiescent position of these bars, i.e. their positions as they await the insertion of the next sheet, the pusher bar is elevated above the stack. The movement of this bar is essentially oscillatory-linear, in a generally vertical direction. Thus, this bar can be stored in a minimum size cavity, in a continuous wall extending between the sheet's entry path and the above-mentioned wire form. In this manner, catching of the sheet's leading edge, as for example due to electrostatics, is minimized.
The hold-down bar, in its quiescent position, rests on top of and holds down the stack's trailing edge portion, so as to prevent obstruction of the next sheet to be inserted.
After this sheet has been inserted, its trailing edge resides between the two bars. The pusher bar now descends vertically onto this sheet, at a point between the hold-down bar and the sheet's leading edge, and pushes its generally trailing edge portion down onto the stack. The hold-down bar now begins a generally circular movement. The first portion of this movement is generally in the plane of the stack's top sheet, and toward its trailing edge. This movement continues until the hold-down bar has moved far enough to clear the trailing edge of the largest size sheet to be stacked. The hold-down bar now moves up, back over the stack, and down onto the stack, to its quiescent position. The pusher bar has by this time returned to its elevated quiescent position.
The above-described movement of the present invention's two-bar mechanism allows the sheet stacking apparatus to function in the small space required for optimum sheet stacking from a fixed sheet entry path to a stack of fixed height.
The foregoing and other features and advantages of the invention will be apparent from the following more particular description of a preferred embodiment of the invention, as illustrated in the accompanying drawing.